Aluminum metaphosphate (ALMP), Al(PO3)3, is produced as a calcined product with an aggregate particle size range of 0.1 mm to greater than 5 mm. FIG. 1 shows an example of ALMP particle sizes as calcined. Once calcined, ALMP is a refractory material with a particle hardness (up to approximately 10 GPa). Prior to use of calcined ALMP in a glass batch, ALMP is subjected to particle size reduction via comminution methods using particle fracture mechanisms such as crushing, impact, and attrition.
Impact milling of ALMP is currently carried out in an abrasive metal cylinder, with rings and/or rotor blades within the cylinder for crushing the particles. However, due to the refractory nature of calcined ALMP, the rings and/or rotor blades get worn out during the milling operation, resulting in metal, primarily iron, contamination in the final ALMP particulate product. For glass production, the iron contamination in the final ALMP particulate product is detrimental to final glass product performance attributes. Magnetic separation is usually used as a post solution step to remove metal contaminants from the milled product.
In addition, as a result of calcining, ALMP aggregates are under varying degrees of compression based on aggregate size. FIG. 2 shows an image particle size of ALMP as calcined. Due to this unique microstructure of ALMP, it is difficult to control the desired final particle size distribution. If the input energy to the impact mill is larger than the aggregate compressive stress, the aggregates will be pulverized, producing an ultra-fine ALMP product that may be unsuitable for some glass batch processes, i.e., the ALMP product may be too fine for ease of conveyance and may aggravate batch dusting.